WASHINGTON — Emotions erupted in a contentious Senate hearing on immigration reform Monday during a heated exchange over the Boston bombing.

Opponents of a bipartisan Senate immigration bill said the Boston Marathon tragedy raised questions about the current system and urged restraint in moving ahead with an overhaul of the nation’s immigration laws.

But Senate Judiciary Committee Chairman Patrick Leahy, a Vermont Democrat, urged lawmakers to refrain from exploiting the Boston tragedy for legislative gain on an issue that affects millions of employers, workers and immigrants.

“Let no one be so cruel as to use the heinous acts of two young men last week to derail the dreams and futures of millions of hardworking people,” Leahy said.

The ranking Republican, Sen. Charles Grassley of Iowa, said last week that the Boston bombing showed it was “important to understand the gaps and loopholes in our immigration system.”

“This is not something to be rushed,” Grassley said.

Sen. Charles Schumer, D-N.Y., an author of the bill, issued a rebuke Monday, saying the tragedy in Boston should not be used “as an excuse for not doing a bill or delaying it many months or years.”

That prompted an outburst by Grassley: “I never said that! I never said that!”

Leahy pounded his gavel to stop the verbal confrontation from escalating, but the exchange laid bare the emotions underlying the issue of immigration reform.

Schumer said his comments were not directed at Grassley, or any individual lawmaker, but toward sentiment to delay proceedings on the bill.

He said specific measures in the bill could strengthen security and “make a Boston less likely.”

The two immigrant Chechen brothers believed responsible for the attacks were in this country legally, and one became a naturalized U.S. citizen in 2012.

Homeland Security Secretary Janet Napolitano, scheduled to appear before the panel last week, canceled her testimony as events in Boston unfolded. She is now scheduled to testify before the committee today.

Texas Sens. John Cornyn and Ted Cruz, both Republicans, also missed the committee hearing last week to be back in their home state with victims of the fertilizer plant explosion in the town of West.

In their appearance at Monday’s hearing they thanked lawmakers for their condolences and prayers.

The two Texas lawmakers also signaled their opposition to key portions of the bill, including a path to citizenship for 11 million unauthorized immigrants, contingent upon increased border security.

Cornyn said the bill would need substantial improvement on border security measures to receive his support.

“I regret that the border security elements fall well short of the sponsors’ aspiration to protect the borders and maintain U.S. sovereignty,” Cornyn said. “Without major changes, the bill could do more harm than good.”

The bill calls for $6.5 million in new spending on border fencing, technology and 3,500 additional Border Patrol agents.

It requires the Department of Homeland Security to provide a plan within six months of the bill’s implementation detailing how it will strengthen security measures and increase apprehensions.

In addition to the security measures, Cruz also objected to the plan to provide eventual citizenship to immigrants who overstayed visas or entered this country illegally.

Cruz said the Senate should focus on measures of the bipartisan bill that enjoy consensus from lawmakers in both parties to pass legislation.

“The bill includes elements that are deeply divisive, and none more divisive than a path to citizenship,” Cruz said.

Sen. Lindsey Graham, R-S.C., another author of the bipartisan bill, said failing to address the 11 million people in this country illegally is a de facto amnesty.

“The 11 million are not going away,” Graham said.

Meanwhile, Texas native Arturo Rodriguez, the United Farm Workers of America president, said agricultural provisions in the bill would provide growers with sufficient workers while giving protections to those who work in the fields.

He said more than 600,000 agriculture workers are currently U.S. citizens or permanent legal residents.

The bill would allow growers to bring in agriculture workers but also allow those new workers to apply for citizenship after five years.

Rodriguez said the deal was forged in negotiations brokered by Sen. Dianne Feinstein, D-Calif., who brought the various agricultural interests together behind closed doors.

“I am proud that both Texas senators are on this committee, and I hope to leave here today knowing that I can count on the support of Senators Cornyn and Cruz to advance this proposal,” Rodriguez said.